1. Field of the Invention
The present invention relates to an inverter apparatus, and in particular, relates to a technique for optimally driving gates of power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) within an inverter apparatus.
2. Description of the Related Art
A gate drive circuit that drives gates of power MOSFETs within an inverter apparatus typically includes a charge circuit and a discharge circuit. The charge circuit is used to charge and pull up the gates of the power MOSFETs, and the discharge circuit is used to discharge and pull down the gates.
In a typical inverter configuration, a resistance element and a diode are serially connected along a charging/discharging path between a charge/discharge circuit and a gate of a power MOSFET. The resistance element is used to adjust the time constant of the charge/discharge operation, while the diode is provided to prevent the charge/discharge current from flowing back. Control of the time constant of the charge/discharge operation is important to provide a desired on-time and off-time for the power MOSFET and to thereby adjust a dead time optimally. For example, Japanese Laid Open Patent Application No. JP-A-Heisei 10-313242 discloses an inverter apparatus having a resistance element and a diode serially connected along a path used for charging a gate of a power MOSFET, and also having a resistance element along a path used for discharging the gate. Further, Japanese Laid Open Patent Application JP-P2002-223157A discloses an inverter apparatus having a resonance inductor and a diode serially connected along a path through which a gate is discharged.
One requirement on the inverter apparatus is the control of EMI (electromagnetic interference). One cause of the EMI is resonance between a drain-source capacitance of the power MOSFET and an interconnection inductance, which may occur when the power MOSFET is turned off. Such resonance undesirably develops a resonance current of a high frequency, generating significant EMI. A description is given below on a resonance between the drain-source capacitance and the interconnection inductance
With reference to FIG. 1, which shows a typical configuration of an output stage of an inverter apparatus, the resonance between the drain-source capacitance and the interconnection inductance is related to recovery currents of body diodes 103 and 104, which are incorporated in power MOSFETs 101 and 102. For example, a case is considered where the power MOSFET 101 is switched from the on-state to the off-state, and the power MOSFET 102 is switched from the off-state to the on-state thereafter. When the power MOSFET 102 is switched to the on-state after the power MOSFET 101 is switched to the off-state, the body diode 103 exhibits reverse recovery, and a recovery current flows through the body diode 103. The recovery current causes the resonance between the drain-source capacitance of the power MOSFET 101 and the inductance of an interconnection connected to the power MOSFET 101, leading to the flow of the resonance current of a high frequency.
There is a need for a technique that reduces the EMI caused by the resonance between the drain-source capacitance of the power MOSFET and the interconnection inductance.